1. Field of the Invention
This invention concerns an output circuit. More particularly, this invention concerns an output circuit which is used in an output stage where a large current and a large output amplitude are required, e.g., in a motor drive circuit.
2. Description of the Prior Art
Convention ally, a switching type output circuit or an analogue drive type output circuit has been used as an output circuit in a Output stage Of a brush-less motor drive circuit. In the switching type output circuit, an electric noise occurs. For reducing the noise and for obtaining a smooth operation of the motor, a large capacity capacitor is required. Thus, this switching type output circuit is not desireable from the stand point of size or cost. Therefore, to avoid the above defects, an analogue drive type output circuit is widely used.
FIG. 1 shows an example of an analogue drive type output circuit. In the circuit, a pull-up transistor Q10 and a pull-down transistor Q12 are controlled by an output signal of an amplifier circuit 11. A loop, including diodes D1 to D4 and transistors Q8 to Q11, is formed to control the current of the transistor Q10 when no input current is supplied to the input terminal T1 Namely, the current which flows in the transistor Q10 is controlled by changing the emitter area of each transistor. When a positive input current Ii+ is supplied to the input terminal T1, the input voltage Vin at the non-inverting input terminal is expressed as follows: EQU Vin=Vref+(Ii+).times.R2 (1)
wherein, Vref is the reference voltage supplied to a reference terminal. Thus, the Output of the amplifier circuit 11 becomes high level. There.+-.ore, the current supplied by the current mirror circuit, including a current source I and PNP transistors Q5 and Q6, is supplied to the base electrode of the transistor Q9. Thus, the transistors Q9 and Q10 change to the ON state. In this condition, the transistors Q11 and Q12 are in the OFF state. When a negative current I- is withdrawn from the input terminal T1, the input voltage Vin at the non-inverting input terminal of the amplifier il is expressed as follows: EQU Vin=Vref-(Ii-).times.R2 (2)
Thus, the output of the amplifier becomes a low level. In this state, the transistors Q11 and Q12 become ON state, and the transistors Q9 and Q10 are in the OFF state, respectively. This circuit as a whole forms an amplifier circuit. The relationship between the input current Ii and the output voltage Vo is expressed as follows: EQU R2.Ii=R3.Vo/(R3+R4) (3)
Thus the gain of this circuit is expressed as follows: ##EQU1## By setting R2=R3, the gain G is expressed as follows: EQU G=R3+R4 (5)
wherein R2, R3 and R4 are resistance values of the resistors R2, R3 and R4, respectively. In this way, an analogue drive type output circuit which supplies a large amplitude output voltage and a large current, is achieved. Thus, the electric noise created by the prior art switching type circuit is prevented, and a stabilized output is achieved. Furthermore, a sufficient output current, expressed as follows, is achieved: EQU Ic10=Ic6.times.hfe9.times.hfe10 (6) EQU Ic12=Ic7.times.hfe8.times.hfe12 (7)
wherein Ic10 and Ic12 are the collector currents of transistors Q10 and Q12, and hfe8, hfe9, hfe10 and hfe12 are the current gains of the transistors Q8, Q9, Q10 and Q12, respectively. The maximum output voltage Vmax at the output terminal T2 in the saturation state is expressed as follows: EQU Vmax=Vcc-Vbe10Vbe9-Vces6 (8)
wherein the Vces6 is the collector-emitter voltage of the transistor Q6 in the saturation state thereof. The minimum output voltage Vmin is expressed as follows: EQU Vmin=Vces12 (9)
wherein the Vces12 is the collector-emitter voltage of the transistor Q12 in the saturation state thereof. As Vbe and Vces are about 0.7 (V) and 0.3 (V), respectively, the maximum output voltage Vmax is lowered by about 1.7 (V) with respect to the power source voltage Vcc. Recently, the requirement for lowering the power source voltage has been become more strict. For example, it is necessary to operate the output circuit at a power source voltage of 5 (V). e.g., in a floppy disk drive circuit. Thus, an output circuit which can supply an output current and an output voltage amplitude or a torque as large as possible, is required to sufficiently drive such a motor.